1. Field of the Invention
The present invention relates to an optical pickup apparatus, and more particularly, to an optical pickup apparatus in which the productivity and efficiency of light are improved by reducing the number of parts thereof.
2. Description of the Related Art
FIG. 1 shows a conventional compatible optical pickup apparatus which records/reproduces data with respect to a disc. The optical pickup apparatus includes a first light holder 105 and a second light holder 115. The first light holder 105 includes a first light source 100 which emits a light beam having a wavelength of about 650 nm and a first grating 103 which divides the light beam from the first light source 100 into three light beams to facilitate a servo tracking or a servo focusing. The first light source 100 and the first grating 103 are integrally formed. The second holder 115 includes a second light source 110 which emits a light beam having a wavelength of about 780 nm and a second grating 113 which divides the light beam from the second light source 110 into three light beams to facilitate a servo tracking or a servo focusing. The second light source 115 and the second grating 113 are integrally formed.
The first light holder 105 and the second light holder 115 are independently arranged at different positions. The first light source 100 is used for a disc 107 such as a DVD having a relatively thin thickness while the second light source 100 is used for a disc 117 such as a CD having a relatively thick thickness.
The light beam emitted from the first light source 100 is reflected by a first beam splitter 120, passes through a second beam splitter 123, and proceeds toward the relatively thin disc 107. Then, the light beam is reflected by the relatively thin disc 107, passes through the first and second beam splitters 120 and 123, and is received by a photodetector 130.
A reflection mirror 125 which changes paths of the light beams emitted from the first and second light sources 100 and 110, a collimating lens 127 which makes an incident light beam parallel, and an objective lens 129 which focuses the incident light beam onto the disk 107/117 are provided on an optical path between the second beam splitter 123 and the disc 107/117.
The light beam emitted from the second light source 110 passes through the second grating 113, is sequentially reflected by the second beam splitter 123 and the reflection mirror 125, and passes through the collimating lens 127 and the objective lens 129, thus forming a light spot on the relatively thick disc 117. Then, the light reflected by the relatively thick disc 117 passes through the objective lens 129 and the collimating lens 127, is reflected by the reflection mirror 125, passes through the second and first beam splitters 123 and 120, and is received by the photodetector 130.
Here, the first and second beam splitters 120 and 123 respectively split the light beams emitted from the first and second light sources 100 and 110 into approximately 50:50 and use only 50% of the received light, so that the efficiency of light is very low.
An astigmatism lens 132 is provided between the first beam splitter 120 and the photodetector 130. The astigmatism lens 132 does not have a uniform curvature, but has different curvatures in vertical and horizontal directions to generate an astigmatism. The astigmatism lens 132 is arranged at an angle in a direction opposite to a direction in which the first beam splitter 120 is inclined, such that the size of the light beam focused on the photodetector 130 is increased by the interaction with the collimating lens 129, and a coma aberration generated with respect to the light beam that passes through the first beam splitter 120, is increased. Also, the curvatures of a lens surface in the vertical and horizontal directions are formed to be different from each other to generate the astigmatism. Here, a focusing error is detected in an astigmatism method by using the astigmatism generated as described above.
The light beams emitted from the first and second light sources 100 and 110 are respectively divided into three light beams by the first and second gratings 103 and 113. A focusing error is detected by using the three light beams in a differential push-pull method with respect to the relatively thin disc 107 and in a three-beam method or push-pull method with respect to the relatively thick disc 117. Since the differential push-pull method, the three-beam method, and the push-pull method are well-known techniques in the art, detailed descriptions thereof will be omitted herein.
Accordingly, CDs and DVDs can be compatibly recorded/reproduced by a single pickup apparatus having the above structure. However, since the conventional optical pickup apparatus has a separate light source and a separate grating for a CD and for a DVD, as well as two beam splitters, the number of parts increases. Therefore, the cost is raised, and portions of the pickup apparatus require numerous adjustments for an optical alignment. Accordingly, the productivity is lowered and a fraction defective is relatively high compared to a case having less number of parts. In other words, parts adopted in an optical pickup apparatus are designed according to a focal distance and optical length of a lens, and positions of the respective optical parts are determined accordingly thereafter. Here, an allowance in design or manufacture is unavoidably generated at the respective parts. Also, as the number of parts increases, allowance of each part increases. Thus, a light beam emitted from a light source in an above optical pickup apparatus does not accurately focus on a disc, thereby deteriorating a sensitivity of a signal thereof. Furthermore, where the light beam focuses on the disc, asymmetrically, a difference in the quantity of light is generated according to the position of the light beam so that the light beam focused on a photodetector becomes asymmetric and jitter increases.
Additionally, a motor which rotates a disc and chips may be presented in a layer where the optical pickup apparatus is installed. Thus, heat is generated from the motor or chips during a reproduction of a disc. In some cases, the internal temperature rises up to 60° C. even though a fan to cool the heat is installed. However, since parts inside the conventional pickup apparatus are attached by a UV bond, a portion attached by the UV bond is twisted or bulged by the high temperature. Accordingly, positions of the optical parts change and they are deviated from an optical axis so that the signal reproduction performance is lowered. Thus, it is necessary to reduce the portions, where the parts are attached by the UV bond, by reducing the number of parts in the optical pickup apparatus.
Also, a reflectance ratio of a light beam of a recordable disc is low due to its material, compared to a read-only disc. Accordingly, since the quantity of light that is reflected is small, it is disadvantageous in terms of detecting a signal, and the signal detection is affected more by noise. Thus, a light source used for a recordable disc must have a higher power than that of a light source used for a read-only disc. To increase optical power, as shown in FIG. 2B, a single mode laser light source is used. However, while the single mode laser light source may be effective in increasing the optical power, it is disadvantageous in terms of removing the effect by noise.
To reduce the noise, as shown in FIG. 2A, a multi-mode laser light source is used. A multi-mode laser light source having a high optical power for use as a laser light source having a wavelength of 780 nm has been developed. However, presently, it is difficult to manufacture a multi-mode laser light source having a high power for use as a laser light source having a wavelength of 650 nm. To solve the above problem, an HFM (high-frequency modulation), that is, a high frequency apparatus, is used for a multi-mode. However, where the high frequency apparatus is used, since a high frequency is dangerous to a human body, an electromagnetic shielding apparatus is necessarily added to protect the human body from the high frequency. This in turn increases the number of parts, and the dimension of an apparatus having the same is increased. Furthermore, where care is not taken to completely shield the high frequency, a user may be exposed to a very dangerous situation.
In addition, where a disc having a large birefringence is reproduced, since the conventional optical pickup apparatus does not have an apparatus to reduce a change in polarization according to a birefringence, a reproduction performance thereof is significantly deteriorated.